1. Field of the Invention
The present invention relates to a switching mode power supply (SMPS). More particularly, the present invention relates to a quasi-resonant switching type SMPS and a driving method thereof.
2. Description of the Related Art
An SMPS is a device that rectifies an input AC voltage to an input DC voltage (DC-link voltage) and converts the input DC voltage to an output DC voltage having a different level. The output DC voltage can be higher or lower than the input DC voltage. The SMPS is generally used for battery supplies that power electronic devices, in particular, digital televisions and computer displays.
A quasi-resonant SMPS turns on a main switch at valleys of the drain-source voltage (Vds) of the switch.
FIG. 1 is a diagram illustrating a relationship between the switching frequency fs and the output power (Po) of a conventional quasi-resonant SMPS when the input AC voltage is 110V or 220V.
FIG. 1 illustrates that the output power Po of the SMPS decreases as the switching frequency fs increases. Accordingly, as Po decreases, switching losses increase. As also shown, when the AC input voltage Vin increases, the switching frequency fs further increases. As a result, there may be an increase in switching losses and audible noise due to intermittent switching.
FIG. 2 illustrates distributions of the input power limit PinLim and the switching frequency fs as a function of the input voltage Vin of a conventional quasi-resonant SMPS. PinLim depends upon the current limit ILim of the SMPS. In the operation of the conventional quasi-resonant SMPS, a current flowing from the drain to the source of the main switch Ids is prevented from exceeding ILim. Accordingly, the input power Pin of the SMPS is limited not to exceed a predetermined level, the input power limit PinLim, to prevent excessive power input.
However, even when Ids is limited to the current limit ILim, the input power Pin is not necessarily limited to PinLim. As shown in FIG. 2, as Vin increases, the switching frequency fs can increase and the input power limit PinLim increases. Accordingly, there may be excessive power input into the SMPS, placing stress on the main switch, possibly damaging the switch.
A conventional quasi-resonant SMPS can solve this problem by turning on the main switch at a minimum of a second valley of the Vds voltage when the switching frequency fs exceeds a reference frequency. This is described with reference to FIG. 3.
FIG. 3 is a diagram illustrating the switching of the main switch of a conventional quasi-resonant SMPS at a minimum of a minimum of a first valley or minimum of a second valley of the Vds voltage, depending upon the output power Po and the switching frequency fs.
As shown in FIG. 3, the conventional quasi-resonant SMPS turns on the main switch at a minimum of a second valley of the Vds voltage if the switching frequency fs is greater than a reference frequency due to a small output power Po. However, the SMPS turns on the main switch at a minimum of a first valley of the Vds voltage if the switching frequency fs is smaller than a reference frequency due to a large output power Po. Accordingly, the SMPS turns on the main switch at a minimum of a second valley of the Vds voltage when output power Po is small to prevent the switching frequency fs from exceeding a predetermined level, thereby protecting the switch from excessive power input and stress.
However, due to a change of the output power Po in the conventional quasi-resonant SMPS, a ripple is generated in the output voltage Vo when the turn-on time of the main switch changes from a minimum of a minimum of a first valley of the Vds voltage to a minimum of a second valley of the Vds voltage, or when the turn-on time of the main switch changes from a minimum of a second valley of the Vds voltage to a minimum of a minimum of a first valley. In particular, the ripple generates noise on a screen when the SMPS is used for an image display device such as a cathode ray tube (CRT) TV. The ripple may be even more pronounced for a high definition television (HDTV).
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.